Cover Page for Lab Report Group Portion. Lift on a Wing

Transcription

1 Cover Page for Lab Report Group Portion Lift on a Wing Prepared by Professor J. M. Cimbala, Penn State University Latest revision: 17 January 2017 Name 1: Name 2: Name 3: [Name 4: ] Date: Section number: ME 325. Group # Score (For instructor or TA use only): Lab experiment and results, plots, tables, etc. - Procedure portion Discussion Neatness & grammar TOTAL / 45 / 15 / 10 / 70 Comments (For instructor or TA use only):

2 Procedure and Presentations of Results A. Calibration of the electronic pressure transducer The two most common devices for measuring small differences in pressure are liquid manometers and electronic pressure transducers. When a pitot-static probe is used to measure flow velocity, either of these devices can be used. The water manometer used in this experiment has an inclined portion which permits accurate positioning of the meniscus. When used properly, the inclined micromanometer in the Fluids Lab has an accuracy of ±0.001 inches of water. The water in the manometer is actually a colored mixture which has a specific gravity of 1.000, i.e. its density is kg/m 3. The electronic pressure transducer is a device that provides a voltage output directly proportional to the pressure difference. For velocity measurements with a pitot-static probe, the electronic transducer is preferred over a liquid manometer for several reasons. First, its response time is much faster. Second, its output can be read directly from a digital voltmeter display. Measurements are much easier because no adjustments are necessary once the unit is calibrated. Finally, since the output is a voltage, it can easily be connected to a computer-controlled data acquisition system. In this lab experiment, an electronic pressure transducer is used for all flow measurements. However, it is first necessary to calibrate the transducer. The Validyne transducer used in this experiment can be adjusted to any desired ratio of voltage output to inches of water. Normally, the unit is adjusted so that 1.0 Volts on the display corresponds to 1.0 inches of water column pressure. However, the pressure difference encountered by the pitot-static probe in our wind tunnel can exceed 5.0 inches of water column; meanwhile, the computerized data acquisition system is set up for inputs in the range of -5.0 to 5.0 Volts. (Voltages above or below these limits are clipped.) Therefore, in order to avoid clipping by the data acquisition system, we will calibrate the transducer such that 1.0 Volts corresponds to a pressure of 4.0 inches of water column. Use the following procedure to calibrate the electronic pressure transducer: 1. First, the manometer must be leveled. Inspect the two level bubbles on the base of the instrument. If the bubbles are not exactly within the hairline marks, use the leveling screws on the base to level the instrument. 2. Crank down the water manometer so that the fluid level is at exactly 4.00 inches. Use the round fine-adjustment scale near the bottom of the manometer for greatest accuracy. 3. Connect the hand pump to the manometer and to the pressure transducer as sketched in Figure 3. To avoid leaks, make sure that the black rubber gaskets are in place in the hose connectors. 4. Make sure the wind tunnel is off, and the pressure release on the hand pump is open (pushed in). Under these conditions there is zero pressure difference across the manometer and across the transducer head (P = P atm at both ports). 5. With the Validyne transducer switched to the Lo range, adjust the Zero adjust potentiometer to achieve a zero reading. 6. Close (release) the pressure release valve on the hand pump, and pump slowly until the liquid level is again exactly at the zero mark on the inclined portion of the water manometer. Use the blue thumb valve to completely seal the system when at the zero mark. Sometimes the small black washer gasket on the blue quick connect falls out and causes a leak. Check for leaks. At this point, there is exactly 4.00 inches of water pressure (head) on the high (+) side of the electronic pressure transducer head, with the low (-) side open to atmospheric pressure. 7. With the Validyne transducer still set on the Lo range, adjust the Span adjust potentiometer to achieve a reading of 1.00 volts. The electronic pressure transducer is now calibrated to read exactly 1.00 volts per 4.00 inches of differential water pressure. 8. Record the potentiometer readings on the Validyne display unit: Meniscus Zero mark Ruler (inches) Inclined manometer tube Pointer Figure 2. Manometer at zero inches of head. 0 1 Zero: Span:

3 Tubing High P atm Low Tee Meriam inclined manometer Pressure transducer P atm Low - + High Quick connector Cable Hi Zero Lo Span Pressure transducer display Hand pump Figure 3. Schematic diagram of how to calibrate the pressure transducer using the inclined manometer. 9. Replace the tee connection on the high (+) side of the transducer head with the total pressure line from the pitotstatic tube. Also, make the connection between the low (-) side of the transducer and the static pressure line from the pitot tube. Measuring the difference between wind tunnel total and static pressures will enable you to monitor the wind tunnel speed. 10. Slowly crank the manometer back up to the zero level so that it is ready for the next group. The water manometer is no longer needed. Carefully return the hand pump to its resting place so that it does not get knocked off. B. Calibration of the lift balance The lift balance installed in this wind tunnel is a cantilever-type balance with strain gages, mounted as shown in Figure 4. Briefly, the strain gages measure the amount of deflection of the cantilever, which is directly proportional to lift force. The Measurements Group strain indicator provides the necessary power supply to the strain gage circuitry. The switches should be set as follows: The Full yellow button should be pushed in The Gage Factor should be set on The Excitation should be dialed to 7 The Range button should be 100% There is also a TSI integrating voltmeter connected to the output. This should be set to DC Volts and 10 V range. The time constant should be at 1 second. This voltmeter will be used to monitor both the calibration of the lift balance and for the lift measurements in the wind tunnel. Its reading will not be the same as that of the strain indicator, but it is linearly proportional. The advantage is that it integrates over time, making it easier to read when there is an unsteady lift. Use the following procedure to calibrate the lift balance: 1. If the computer is off, turn it on. From the Windows desktop or from the Start Menu, start the program ME325- Lift Calibrate. This is a program which will assist you in calibration of the lift balance. As you apply weights to the lift balance calibration rig, the voltage output from the strain indicator is read by the computer. When finished with the calibration, the program then performs a least-squares curve fit of the calibration data, which is then used in the subsequent lift measurements of the various models.

4 2. The two strings that come out below the test section are for use in calibration of the lift balance. The one labeled LIFT should be used for the lift calibration program and the one labeled DRAG should be used for the drag calibration program, used in a different lab. Sting mount Strut 1/4-20 threads Fairing Floor of wind tunnel Strain gages Drag and lift balance Figure 4. Schematic diagram of the lift balance with sting mount. 3. Click on Select File for Saving Data to enter a file name for the output data. By default, all files go to the C:\temp directory. The file must be a text (*.txt) file. Pick a file name that is unique to your group, e.g. include a member s last name, such as Smith_groupC_lift_calib.txt. Now you are ready to calibrate. 4. With no weight on the string, adjust the potentiometer labeled Balance on the strain indicator until the voltmeter reads zero. Enter 0 as the applied mass, and sample the first data point reading of the lift calibration program. NOTE: The strain indicator is a Wheatstone bridge type circuit, which occasionally drifts with temperature. It may be necessary to re-adjust the zero balance on the strain indicator occasionally. 5. Carefully add masses of various values and take more data points. Typically, about ten data points (ranging from 20 grams to 1000 grams) are sufficient for a good calibration, and they do not have to be applied in increasing order. Note: The data acquisition board in the computer is set for a range of -5.0 to 5.0 volts. Monitor the voltage displayed on the TSI integrating voltmeter. Do not exceed 5.0 Volts, or your signal will be clipped, and the reading will be erroneous. 6. When finished, End Calibration Get Best-Fit Line. The computer then calculates a linear least-squares curve fit, and displays the coefficients. Write the calibration coefficients here: Slope = N/V Intercept = N 7. Open the data file that you generated. Check the last line in the file, which should look like this: where 25 is just a place holder here it should be the last row number, depending on how many points you took. If that line is not there, you need to add it; it is required to signify the end of the data. (5) 8. Finally, using the data file you created, generate a plot of the lift balance calibration (lift force as a function of voltage), using Microsoft Excel. Use symbols for the data points, and use a solid line for the curve fit. Ask your instructor or TA for help generating the plot if necessary. Show your plot to your TA or instructor for his/her initials before continuing. Attach the approved plot to your report. Save your data file and your Excel file in case you need them later. See Figure. C. Operation of the Wind Tunnel and the Pitot-Static Probe In this section, you will familiarize yourself with the operation of the wind tunnel, and will practice calculating the wind tunnel velocity from the differential pressure reading of the Pitot-static probe. Be sure that no loose tools or other objects are in the test section. Close and secure the wind tunnel test section door. Before proceeding, make sure the pressure

5 transducer has been calibrated to 1.0 volt per 4.0 inches of water, as described in the previous section. Adjust the zero potentiometer on the Validyne unit, if necessary, since the zero sometimes drifts with temperature. 1. Record the ambient (barometric) pressure from the gauge which is mounted on the wall of the wind tunnel. It is often necessary to lightly tap the casing of the barometer to get a good reading. Be sure to use the proper scale, which is cm of mercury. (Typically in State College, the barometric pressure is around 72 cm of mercury.) P atm = cm of mercury. 2. Record ambient temperature from the glass thermometer located under the barometer. Note: Temperature is also measured by a thermocouple, and is recorded by the digital data acquisition system see Channel 2 of the Techkor unit. To read the temperature, divide the number on the display by 10 this is T in o C (1 volt = 10 o C). Write down both temperatures below they should be close. T ambient, glass thermometer = C T ambient, thermocouple = C. (2) 3. Using the ideal gas law, and the thermocouple temperature, calculate the air density. Show your calculations below. Note: One centimeter of mercury is equivalent to N/m 2 of pressure. The gas constant, R, for air is 287. m 2 /(s 2 K). = kg/m 3. (2) 4. Calculate or look up the coefficient of viscosity,, of the air at the temperature indicated by the thermocouple. = N s/m The speed of the wind tunnel is controlled by a Toshiba Tosvert 130H1 Transistor Inverter. First make sure the remote control unit (mounted to the side of the wind tunnel) is turned to Stop. Turn on the main circuit for the 480 Volt power supply (located on the wall around the end of the wind tunnel). Push in the Start button on the small electric box just below the Toshiba wall-mounted control box. The display should read OFF. From here on, you will use the dial on the remote control to control the wind tunnel speed. (The remote control is located on the wind tunnel below the test section) (5) 6. Flip the bottom switch of the remote control to Manual, set the dial setting to a reading of around 200, and turn on the wind tunnel by rotating the upper switch to Start. Slowly increase the wind tunnel speed, using the dial. As the fan frequency increases, so should the Pitot-static pressure. Make five manual calculations of wind tunnel speed, using Equations (4) and (5), and show your calculations below. The last case should be at maximum wind tunnel speed. Be careful to use the proper SI units, and remember that the Validyne display represents 4.00 inches of water column for every 1.00 volt. Motor frequency (Hz) Validyne display reading (Volts) Pitot-static pressure difference (inches of water column) Velocity (m/s) 7. The maximum wind tunnel speed is m/s. Note: The maximum wind tunnel speed should be in the range between 40 and 60 m/s. If it is not, check your calculations and electronic and pressure line connections. (Often the problem is a leak in one of the blue quick connects or in the pitot-static probe connectors) If the problem is not found, consult your instructor or TA for assistance. 8. Turn the remote control switch to Stop to turn off the wind tunnel. Leave the wall-mounted Toshiba inverter unit on, however, until you are finished with the lab.

6 D. Measurement of Lift on the Model Fin With the aid of the computerized data acquisition system, you will measure the lift on the model fin at various angles of attack, and at various Reynolds numbers. 1. If not already in place, install the model fin to the wind tunnel drag/lift balance. Adjust the angle of attack to 0. Be careful that you don t over-tighten anything or apply too much force on the model. Consult your instructor or TA if necessary to clarify the operation of the equipment. 2. Turn on the Toshiba Tosvert 130H1 Transistor Inverter (mounted on the wall). The display should read OFF. With the wind tunnel off, re-zero both the lift balance and the pressure transducer if necessary. 3. A computer data acquisition program called ME325-Lift Measure is available for this lab. This program is nearly identical to the drag measurement program of the other wind tunnel lab, except it measures lift rather than drag. Consult the instructions of that lab manual if necessary to refresh your memory about using the computer or the wind tunnel. You will need to enter the barometric pressure, length and area scales, etc., as previously. For the length scale, use the chord length of the model fin (c = 5.8 in). For the area, enter the planform area in square inches (A = 56.3 in 2 ). You will also need to enter the name of the lift calibration file that you just created, and a new file name for the lift data. We suggest a descriptive file name that includes one or more of your group members names, and the nominal Reynolds number for this run, e.g., Jones_Watson_Smith_Re_ txt. 4. Adjust the wind tunnel speed while taking data until the Reynolds number is close to 100,000. When the Reynolds number is as close to 100,000 as possible, take several data points (5 are recommended) at this Reynolds number and angle of attack (zero degrees). Record the blower frequency (on the wall-mounted Toshiba frequency controller) and the wind tunnel velocity corresponding to this Reynolds number in the spaces below: Blower frequency and tunnel speed at 0 o and at Re c = 100,000: Frequency: Hz Velocity: m/s. 5. Increase the angle of attack to 2.5 o, using the rotational traverse mounted under the airfoil. Without exiting the Lift Measure program, enter the new angle of attack in the Enter Parameters window. Take several data points, again keeping the Reynolds number as close to 100,000 as possible. Note that as the angle of attack is changed, the wind tunnel dial setting may need to be adjusted slightly in order to maintain the correct Reynolds number. 6. Repeat Step 5 for several angles of attack, in increments of 2.5 o, up to a maximum angle of attack of 30 o. Note that due to temperature changes in the room, etc., the lift balance zero may drift slightly with time. It is advisable periodically (every 5 or 10 minutes) to shut off the wind tunnel and re-zero the lift balance. At the maximum angle of attack, record the blower frequency and wind tunnel velocity corresponding to this Reynolds number: Blower frequency and tunnel speed at 30 o and at Re c = 100,000: Frequency: Hz Velocity: m/s. (10) 7. Terminate the lift measurement program. Create a table of your data file for the nominal Reynolds number Re c 100,000 (Microsoft Excel is recommended). The table should be labeled appropriately and attached to your report. At a minimum, it should contain columns for angle of attack, wind tunnel speed, Reynolds number, lift force, and lift coefficient. Use only the data points that have the proper Reynolds number discard the extraneous data. See Table. (10) 8. Repeat Steps 3 through 7 for a Reynolds number of approximately 350,000. Blower frequency & tunnel speed at 0 o at Re c = : Frequency: Hz Velocity: m/s. Blower frequency & tunnel speed at 30 o at Re c = : Frequency: Hz Velocity: m/s. Generate, print, label, and attach a table of data for this Reynolds number. See Table. (5) Is the blower frequency different at 30 o than that recorded above at 0 o angle of attack? (yes or no). Why or why not? (Give a brief explanation of your observations):

7 9. Turn off the wind tunnel, turn off the Toshiba Tosvert 130H1 Transistor Inverter, and set everything back the way it was when you started the lab. You are now ready to plot lift coefficient as a function of angle of attack. Presentation of the Data Now that you have acquired data at two different Reynolds numbers, and at several angles of attack for both of these Reynolds numbers, you will generate plots from your files so that the data can be analyzed visually. (10) 1. Plot lift coefficient C L versus angle of attack at Reynolds number Re c = 100,000. By plotting these data at the same nominal Reynolds number together on the same plot, you will show some indication of the repeatability of the measurements. Use a scatter plot (data points only) rather than a line plot that connects data points. On the same plot, plot the theoretical lift coefficient as a line (no symbols). See Figure. (5) 2. On the same plot, plot lift coefficient C L versus angle of attack at Re c = 350,000, using different symbols so that you can directly compare the data at the two values of Re c.

8 Discussion (5) 1. Theory predicts that the lift coefficient of a wing or fin increases nearly linearly with angle of attack (at small angles), until the wing stalls. Do your data support this? Does your experimental value of the slope (dc L /dα) agree with theory? Why or why not? (5) 2. Comparing your plots at the two different Reynolds numbers, in what way (if any) does Reynolds number affect the lift performance of this model fin? Why? (5) 3. Predict the performance of the fins on the prototype human powered submarine operating at a speed of 3.0 knots in seawater. Specifically, predict the maximum lift force to be expected from one of these fins, and the angle at which you predict that the fins will stall.